Where to find diamonds in Minecraft: the best Y level
Below Y 0, the underground is Deepslate (stone and deepslate mix between Y 0 and Y 8), so most of your finds will be Deepslate Diamond Ore.

Deepslate Diamond Ore is the form you will meet most, since diamonds peak near the bottom of the world, deep in the deepslate. Like every ore, it needs a pickaxe, and the enchantment on that pickaxe changes what it drops. Mining works the same for every ore; the Iron Ore page explains pickaxe tiers, Fortune and Silk Touch.
It is mined with a pickaxe (at least tier iron) and drops Diamond.
This item has no crafting recipe.
Below Y 0, the underground is Deepslate (stone and deepslate mix between Y 0 and Y 8), so most of your finds will be Deepslate Diamond Ore.
Mined with Silk Touch, the block drops itself; without it, it drops Diamond.
It is mined with a pickaxe of the right tier to get its resource; Fortune multiplies the drop and Silk Touch keeps the block. It is an ingredient in one recipe: Diamond.
The fastest tool is the pickaxe. It needs at least an iron pickaxe.
Placed under a Note Block, Deepslate Diamond Ore gives the Bass drum sound.